The Primrose-Kirby Area in the Southern Athabasca, Alberta, Canada: A Detailed Geological Investigation
Published:January 01, 1987
F. Dekker, C. Visser, P. Dankers, 1987. "The Primrose-Kirby Area in the Southern Athabasca, Alberta, Canada: A Detailed Geological Investigation", Exploration for Heavy Crude Oil and Natural Bitumen, Richard F. Meyer
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The Lower Cretaceous oil sands in the Primrose-Kirby area occur in two superimposed, and geologically very different, reservoir units. The lower reservoir unit is the McMurray Formation, with facies changing from a fully terrestrial, fluvial environment at the base to a marginally marine, deltaic environment with major channels near the top. The Wabiskaw Member of the Clearwater Formation forms the upper reservoir unit, with laterally continuous marine sheet sands. The contact between the McMurray Formation and the Wabiskaw Member marks the transgression of the area, generally from the north, by the boreal sea.
The two reservoirs have very different characteristics, as was revealed by detailed sedimentological, mineralogical, biostratigraphical, and particle size analyses. The channel sands in the McMurray Formation form the most favorable reservoirs because the unconsolidated sands are relatively coarse-grained and well-sorted homogeneous quartzarenites to sublitharenites with no significant amounts of clay minerals or glauconite. The channels are 30-40 m (100-130 ft) thick and have an average bitumen content of 11. 2 wt %. The Wabiskaw sediments are finer grained and more poorly sorted heterogeneous litharenites containing significant amounts of fines. In addition, numerous carbonate-cemented, totally indurated zones, up to 1 m (3 ft) thick, occur. The average bitumen content is relatively low (9. 5 wt %), but the very continuous and predictable thickness of approximately 20 m (65 ft) makes the Wabiskaw a very attractive reservoir.
Both McMurray channel sands and Wabiskaw marine sheet sands with a combined total of 700 million barrels of exploitable bitumen in place in the study area are being tested for performance. The promising preliminary results of both reservoir units indicate that the general Primrose area may provide a significant amount of Alberta’s future oil production.
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Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.